consumerKey or consumerSecret is a null reference
(Nothing in Visual Basic) or an empty string.

Remarks

For GMail 3-legged IMAP/SMTP OAuth you can use "anonymous", "anonymous" key pair or
get a registered site for an e-mail account and specify the keys from Google
"Registration for Web Applications" page.
For GMail 2-legged OAuth you should register and get key pair from Google.